arXiv subjects
Publications and source records attributed to W. C. Haxton.
I describe the current status of the solar neutrino problem, summarizing the arguments that its resolution will require new particle physics. The phenomenon of matter-enhanced neutrino oscillations is reviewed. I consider the implications of current experiments -- including the SuperKamiokande atmospheric and LSND measurements -- and the need for additional constraints from SNO and other new detectors.
We describe a strategy for attacking the canonical nuclear structure problem ---bound-state properties of a system of point nucleons interacting via a two-body potential---which involves an expansion in the number of particles scattering at high momenta, but is otherwise exact. The required self-consistent solutions of the Bloch-Horowitz equation for effective interactions and operators are obtained by an efficient Green's function method based on the Lanczos algorithm. We carry out this program for the simplest nuclei, d and $^3$He, to contrast a rigorous effective theory with the shell model, thereby illustrating several of the uncontrolled approximations in the latter.
The shell model is the standard tool for addressing the canonical nuclear many-body problem of nonrelativistic nucleons interacting through a static potential. We discuss several of the uncontrolled approximations that are made in this model to motivate a different approach, one based on an exact solution of the Bloch-Horowitz equation. We argue that the necessary self-consistent solutions of this equation can be obtained efficiently by a Green's function expansion based on the Lanczos algorithm. The resulting effective theory is carried out for the simplest nuclei, d and 3He, using realistic NN interactions such as the Argonne v18 and Reid93 potentials, in order to contrast the results with the shell model. We discuss the wave function normalization, the evolution of the wave function as the "shell model" space is varied, and the magnetic elastic effective operator. The numerical results show a simple renormalization group behavior that differs from effective field theory treatments of the two- and three-body problems. The likely origin of this scaling is discussed.
In these Canberra summer school lectures we treat a number of topical issues in neutrino astrophysics: the solar neutrino problem, including the physics of the standard solar model, helioseismology, and the possibility that the solution involves new particle physics; atmospheric neutrinos; Dirac and Majorana neutrino masses and their consequences for low-energy weak interactions; red giant evolution as a test of new particle astrophysics; the supernova mechanism; spin-flavor oscillations and oscillations into sterile states, including the effects of density fluctuations; and neutrino-induced and explosive nucleosynthesis.
These lectures cover three topics in neutrino astrophysics. The solar neutrino problem is reviewed, with particular emphasis on the microphysics of the standard solar model, detector responses, the quantum mechanics of the MSW mechanism, and the possibility of spin-flavor oscillations. In the second lecture the electromagnetic properties of Dirac and Majorana neutrinos are discussed. Two classic problems in red giant evolution - the triple alpha reaction and the delay of the He flash by anomalous cooling - are used to illustrate how stellar cooling arguments constrain neutrino properties. Axion and Dirac neutrino mass effects on supernova cooling are also discussed. The third lecture describes the supernova explosion mechanism and the associated nucleosynthesis. The neutrino process and r-process are discussed, including the "neutrino fingerprint" on the latter that supports earlier suggestions that the r-process site is near the mass cut of a type II supernova. The effects of tau neutrino oscillations on the r-process are described briefly. The material is presented at a level suitable for graduate students at the beginning of their research careers.
This talk provides a basic introduction for students interested in the responses of detectors to solar, supernova, and other low-energy neutrino sources. Some of the nuclear physics is then applied in a discussion of nucleosynthesis within a Type II supernova, including the r-process and the neutrino process.
I review a number of the open questions about neutrino properties, critique recent hints of neutrino mass, and discuss one recently proposed neutrino mass matrix to illustrate the direction in which we may be headed. I also present one example of the implications of these new developments for astrophysics.
Nuclei are powerful laboratories for studying fundamental symmetries because they filter and enhance specific interactions. I discuss four examples --- hadronic parity violation, atomic electric dipole moments, precision $β$ decay tests, and nuclear tests of neutrino masses --- to illustrate some of the progress that has been made in the past few years.
I discuss three issues relevant to solar neutrino flux measurements: cross section uncertainties in pp chain reactions, uncertainties in the GALLEX/SAGE response to 7Be and 51Cr neutrinos, and the implications of helioseismology for nonstandard suns with mixed cores. A few comments are also offered on electron neutrino to tauon neutrino oscillations, cosmologically interesting neutrino masses, and recent proposals for supernova neutrino observatories.
The neutrinos from a Type II supernova provide perhaps our best opportunity to probe cosmologically interesting muon and/or tauon neutrino masses. This is because matter enhanced neutrino oscillations can lead to an anomalously hot nu_e spectrum, and thus to enhanced charged current cross sections in terrestrial detectors. Two recently proposed supernova neutrino observatories, OMNIS and LAND, will detect neutrons spalled from target nuclei by neutral and charged current neutrino interactions. As this signal is not flavor specific, it is not immediately clear whether a convincing neutrino oscillation signal can be extracted from such experiments. To address this issue we examine the responses of a series of possible light and heavy mass targets, 9Be, 23Na, 35Cl, and 208Pb. We find that strategies for detecting oscillations which use only neutron count rates are problematic at best, even if cross sections are determined by ancillary experiments. Plausible uncertainties in supernova neutrino spectra tend to obscure rate enhancements due to oscillations. However, in the case of 208Pb, a signal emerges that is largely flavor specific and extraordinarily sensitive to the nu_e temperature, the emission of two neutrons. This signal and its flavor specificity are associated with the strength and location of the first-forbidden responses for neutral and charge current reactions, aspects of the 208Pb neutrino cross section that have not been discussed previously. Hadronic spin transfer experiments might be helpful in confirming some of the nuclear structure physics underlying our conclusions.
The ratio of 18O nuclei to electrons in SuperKamiokande is quite small, about 1:5000. However this nucleus provides two unpaired neutrons and a low threshold for solar neutrino absorption, resulting in an unusually large cross section. The 18O nuclei, together with 17O and deuterium, will produce in excess of 460 scattered electrons per year with energies above the initial SuperKamiokande threshold of 6.5 MeV, or about 6.7% of the total solar neutrino signal, for an electron neutrino flux scaled to the current SuperKamiokande counting rate. We explore whether this might complicate efforts to extract the neutrino spectrum from elastic scattering events and whether there might be any prospect of combining 18O and elastic scattering events to separate charged and neutral current interactions. Under current operating conditions the answers are no, as the Fermi, Gamow Teller, and forbidden contributions to the 18O cross section conspire to produce a nearly isotropic cross section. Thus most of the events are indistinguishable from background. This result is relevant to SuperKamiokande detection of solar antineutrinos, which arise in certain oscillation scenarios.
The 51Cr neutrino source experiments play a unique role in testing overall operations of the GALLEX and SAGE solar neutrino experiments. Recently Hata and Haxton argued that the excited-state contribution to the 71Ga cross section for 51Cr neutrino absorption might not be known reliably, despite forward-angle (p,n) measurements. A large-basis shell model calculation reported here indicates that the unusual situation they envisioned - destructive interference between weak spin and strong spin-tensor amplitudes - does occur for the transition to the first excited state in 71Ge. The calculation provides a counterexample to procedures previously used to determine the 51Cr cross section: the predicted (p,n) cross section for this state agrees with experiment, while the BGT value is well outside the accepted 3-standard-deviation limit. The results argue for a shift in the interpretation of the source experiments: they become more crucial as measurements of the 71Ga detector response to 7Be solar neutrinos, and less definitive as wholly independent tests of 71Ge recovery and counting efficiencies.
I consider phenomenological changes in the standard solar model with the goal of testing recent claims that the solar neutrino puzzle requires new particle physics. The assumption of a steady-state sun producing the correct luminosity and governed by standard microphysics appears to leave only one nonstandard solar model possibility open, slow mixing of the solar core on timescales characteristic of 3He equilibration. The conjecture of such mixing raises striking physics issues connected with the standard model 3He instability and the possibility that the 3He abundance gradient might allow the sun's early convective core to persist. While helioseismology might eventually rule out such a "model," contrary to one recent claim I will argue that the helioseismology of a mixed two-fluid sun is as yet far from clear. Finally, I conclude by stressing that 3He-driven slow mixing is not being proposed as a solution of the solar neutrino problem, but as an example of a possibility that has not been quantitatively modeled and yet could produce neutrino fluxes far closer to experiment than the standard solar model. Thus, despite the attractiveness of neutrino oscillation solutions, astrophysical explanations of the solar neutrino problem are not yet, in my view, ruled out definitively.
I discuss in qualitative terms the implications of the recent measurement by C. S. Wood et al. of the hyperfine dependence of atomic parity violation in 133Cs. The derived anapole moment constrains the strength of hadronic parity violation.
The opening of the meson factories twenty years ago provided nuclear physics with new beams, higher momentum transfers, and new opportunities for precision measurements. The resulting changes in nuclear physics were substantial, altering not only the range of physics issues identified with the field but also the manner and size of the collaborations that do nuclear physics. Inspired by the talks of this symposium, I discuss some of the accomplishments as well as some of the goals not yet reached.
If the r-process occurs deep within a Type II supernova, probably the most popular of the proposed sites, abundances of r-process elements may be altered by the intense neutrino flux. We point out that the effects would be especially pronounced for 8 isotopes that can be efficiently synthesized by the neutrino reactions following r-process freeze-out. We show that the observed abundances of these isotopes are entirely consistent with neutrino-induced nucleosynthesis, strongly arguing for a supernova r-process site. The deduced fluences place stringent constraints on the freeze-out radius and dynamic timescale of the r-process.
In order to explore the consequences of the neutrino irradiation for the supernova r-process nucleosynthesis, we calculate the rates of charged-current and neutral-current neutrino reactions on neutron-rich heavy nuclei, and estimate the average number of neutrons emitted in the resulting spallation. Our results suggest that charged-current $ν_e$ captures can be important in breaking through the waiting-point nuclei at N=50 and 82, while still allowing the formation of abundance peaks. Furthermore, after the r-process freezes out, there appear to be distinctive neutral-current and charged-current postprocessing effects. A subtraction of the neutrino postprocessing effects from the observed solar r-process abundance distribution shows that two mass regions, A=124-126 and 183-187, are inordinately sensitive to neutrino postprocessing effects. This imposes very stringent bounds on the freeze-out radii and dynamic timescales governing the r-process. Moreover, we find that the abundance patterns within these mass windows are entirely consistent with synthesis by neutrino interactions. This provides a strong argument that the r-process must occur in the intense neutrino flux provided by a core-collapse supernova.
Recent solar neutrino experiments have shown that both $ϕ(^8$B) and the neutrino flux ratio $ϕ(^7$Be)/$ϕ(^8$B) are substantially below their standard solar model values, leading some to discount the possibility of an astrophysical solution to the solar neutrino puzzle. We test this conclusion phenomenologically and find that the discrepancies can be significantly reduced by a distinctive pattern of core mixing on timescales characteristic of $^3$He equilibration.